impervious (to) vs phlegmatic

impervious (to)

phlegmatic

noun
  • One who has a phlegmatic disposition. 

adj
  • Generating, causing, or full of phlegm. 

  • Not easily excited to action or passion; calm; sluggish. 

How often have the words impervious (to) and phlegmatic occurred in a corpus of books? (source: Google Ngram Viewer )